技术iOS学习

深拷贝、浅拷贝的理解与使用场景

2017-10-30  本文已影响680人  Leafly

什么是深拷贝、浅拷贝?

通俗解释:深拷贝是内容拷贝,浅拷贝是地址拷贝

区别点:

深拷贝会创建一个新的内存空间,拷贝的值是一样的,但是内存地址不一样。
浅拷贝只是拷贝指向原来对象的地址,使原对象的引用计数+1

容器类对象与非容器类对象在深拷贝、浅拷贝方面的异同点?

什么是容器类对象?什么是非容器类对象?

像NSString、NSNumber这些不能包含其他对象的叫做非容器类对象
像NSArray、NSDictionary这些可以包含其他对象的叫容器类对象

不可变对象NSString

NSString *string1 = @"helloworld";  
NSString *string2 = [string1 copy]; // 浅拷贝  
NSString *string3 = [string1 mutableCopy]; // 深拷贝  
NSMutableString *string4 = [string1 copy]; // 浅拷贝  此处需要注意copy返回的是不可变对象,string4不能被修改 否则会发生崩溃
NSMutableString *string5 = [string1 mutableCopy]; // 深拷贝  
  
NSLog(@"string1 = %d;string2 = %d",string1,string2);  
NSLog(@"string1 = %d;string3 = %d",string1,string3);  
NSLog(@"string1 = %d;string4 = %d",string1,string4);  
NSLog(@"string1 = %d;string5 = %d",string1,string5);  

打印结果如下:

通过对比不难发现:

可变对象NSMutableString

上面我们使用的是不可变的NSString,下面我们再使用可变的NSMutableString对比一下:

// 如果是一个MutableString,那么无论是copy,mutableCopy,都会创建一个新对象。  
   NSMutableString *string1 = [NSMutableString stringWithString:@"helloworld"];  
   NSString *string2 = [string1 copy]; // 深拷贝  
   NSString *string3 = [string1 mutableCopy]; // 深拷贝  
   NSMutableString *string4 = [string1 copy]; // 深拷贝  此处需要注意copy返回的是不可变对象,string4不能被修改 否则会发生崩溃
   NSMutableString *string5 = [string1 mutableCopy]; // 深拷贝  
  
   NSLog(@"string1 = %d;string2 = %d",string1,string2);  
   NSLog(@"string1 = %d;string3 = %d",string1,string3);  
   NSLog(@"string1 = %d;string4 = %d",string1,string4);  
   NSLog(@"string1 = %d;string5 = %d",string1,string5);  

打印结果如下:

不难发现,对于NSMutableString, 无论是copy还是mutableCopy都会创建一个新对象,属于深拷贝

不可变对象NSArray

NSArray *array01 = [NSArray arrayWithObjects:@"a",@"b",@"c", nil nil];  
NSArray *copyArray01 = [array01 copy];  
NSMutableArray *mutableCopyArray01 = [array01 mutableCopy];  
   
NSLog(@"array01 = %d,copyArray01 = %d",array01,copyArray01);  
NSLog(@"array01 = %d,mutableCopyArray01 = %d",array01,mutableCopyArray01);  
  
NSLog(@"array01[0] = %d,array01[1] = %d,array01[2] = %d",array01[0],array01[1],array01[2]);  
NSLog(@"copyArray01[0] = %d,copyArray01[1] = %d,copyArray01[2] = %d",copyArray01[0],copyArray01[1],copyArray01[2]);  
NSLog(@"mutableCopyArray01[0] = %d,mutableCopyArray01[1] = %d,mutableCopyArray01[2] = %d",mutableCopyArray01[0],mutableCopyArray01[1],mutableCopyArray01[2]);  

打印结果如下:

不难发现,copy是浅拷贝,mutableCopy是深拷贝,不过需要注意的是容器对象的成员元素都指向相同的地址

可变对象NSMutableArray

NSMutableArray *array01 = [NSMutableArray arrayWithObjects:@"a",@"b",@"c", nil nil];  
NSArray *copyArray01 = [array01 copy];  
NSMutableArray *mutableCopyArray01 = [array01 mutableCopy];  

NSLog(@"array01 = %d,copyArray01 = %d",array01,copyArray01);  
NSLog(@"array01 = %d,mutableCopyArray01 = %d",array01,mutableCopyArray01);  
NSLog(@"array01[0] = %d,array01[1] = %d,array01[2] = %d",array01[0],array01[1],array01[2]);  
NSLog(@"copyArray01[0] = %d,copyArray01[1] = %d,copyArray01[2] = %d",copyArray01[0],copyArray01[1],copyArray01[2]);  
NSLog(@"mutableCopyArray01[0] = %d,mutableCopyArray01[1] = %d,mutableCopyArray01[2] = %d",mutableCopyArray01[0],mutableCopyArray01[1],mutableCopyArray01[2]);  

打印结果如下:

对比可见,容器对象与非容器对象类似,可变对象的复制都是深拷贝,不可变对象copy是浅拷贝,mutableCopy是深拷贝
需要注意的是对容器而言,元素对象始终是指针复制

如何实现容器对象的完全深拷贝?

正如前面所说,容器对象中的元素对象无论是copy还是mutableCopy都是指针复制,如何实现容器对象的完全深拷贝呢?

系统API

系统为我们实现容器对象的完全深拷贝提供了方法

- (instancetype)initWithArray:(NSArray<ObjectType> *)array copyItems:(BOOL)flag

// 使用方式如下:

- (void)fullCopy {
    NSMutableArray *marray1 = [[NSMutableArray alloc] init];
    
    NSMutableString *mstr1 = [[NSMutableString alloc]initWithString:@"value1"];
    NSMutableString *mstr2 = [[NSMutableString alloc]initWithString:@"value2"];
    
    [marry1 addObject:mstr1];
    [marry1 addObject:mstr2];
    
    NSArray *marray2 = [[NSArray alloc] initWithArray:marry1 copyItems:YES];
    
    NSLog(@"marry1:%p - %@ \r\n",marry1,marry1);
    NSLog(@"marry2:%p - %@ \r\n",marray2,marray2);
    NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry1[0],marry1[1]);
    NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marray2[0],marray2[1]);
}

归档解档方法

- (void) deplyFullCopy
{
    NSMutableArray *marry1 = [[NSMutableArray alloc] init];
    
    NSMutableString *mstr1 = [[NSMutableString alloc]initWithString:@"value1"];
    NSMutableString *mstr2 = [[NSMutableString alloc]initWithString:@"value2"];
    
    [marry1 addObject:mstr1];
    [marry1 addObject:mstr2];
    
    NSData *data = [NSKeyedArchiver archivedDataWithRootObject:marry1];
    NSArray *marray2 = [NSKeyedUnarchiver unarchiveTopLevelObjectWithData:data error:nil];
    
    NSLog(@"marry1:%p - %@ \r\n",marry1,marry1);
    NSLog(@"marry2:%p - %@ \r\n",marray2,marray2);
    NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry1[0],marry1[1]);
    NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marray2[0],marray2[1]);
}

自定义类如何实现深、浅拷贝

@interface Person()<NSCopying, NSMutableCopying>  
  
@end  
  
@implementation Person  
  
// 对应copy方法  
- (id)copyWithZone:(NSZone *)zone  
{  
    Person *person = [[Person allocWithZone:zone] init];  
    person.name = self.name; // 这里的self就是被copy的对象  
    person.age = self.age;  
    return person;  
}  
  
- (id)mutableCopyWithZone:(NSZone *)zone  
{  
    Person *person = [[Person allocWithZone:zone] init];  
    person.name = self.name;  
    person.age = self.age;  
    return person;  
}  
  
  
@end  

使用场景

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